Problem: $ { {1} \times \left[\begin{array}{r}{2} \\ {3} \\ {-1}\end{array}\right] = {?} } $
Answer: To multiply a matrix by a number, multiply each item of the matrix by the number. ${\left[\begin{array}{r}{1}\times{2} \\ {1}\times{3} \\ {1}\times{-1}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{r}{2} \\ {3} \\ {-1}\end{array}\right]}$